I had a similar problem in my network.
I agree with Boko regarding MHT thing.
I suggest Esamoraglo to change LAC boundry and avoid trains crossing the LAC boundry.
If possible on your part just try increasing no of SDCCH Blocks for a day or 2 and then observe whats the change. If you feel this may bring in TCH blocking, try going in for half rate.
This is what I did and for my 2 TRX problamatic cell, normally we go in for 8 SDCCH Blocks, but I had to finnaly set SDCCH blocks to 24. I also managed to pick up extra traffic on voice front also.
Reason for my recomendation:
SDCCH traffic is 22.12 Erlang.
To cater this with 1% GoS, you need about 32 channels(From Erlan B table), So you need atleast 32 SDCCH Blocks to cater the SDCCH requirement.
I hope it helps